Within the framework of energy saving for the struggle against global warming, different types of so-called low-power-consumption lamps have been developed to replace incandescent lamps wherein more than 90% of the energy consumed is not converted into light. Among the new types of low-power-consumption lamps offered on the market should mainly be mentioned glow discharge lamps, of which the two principal embodiments are commonly called “neon” tubes and compact fluorescent lamps. In general terms, electrode fluorescent lamps are based on the emission of ultraviolet (UV) rays, generated inside a tube that is linear (neon tube) or folded back on itself (compact fluorescent lamp), by a periodic low-frequency discharge (50 or 60 Hz for example), the UV being transformed into visible light by phosphors covering the inside of the tube. The gas mixtures generally used are mixtures of rare gases (mainly argon) seeded with mercury, an active element with principal UV emission lines situated at 254 nm (the biggest line), 297 nm, 313 nm and 365 nm (UVA) (not an exhaustive list), wavelengths at which the fluorescence efficiencies, that is the conversion of photons into visible light on the phosphors lining the inside of the lamps, are highest.
Glow discharge lamps comprise two electrodes (anode and cathode) located at the end of a sealed tube filled with a gas mixture (rare gases and mercury) under low pressure, on the order of a mbar or a torr (1 torr=133 Pascal). The plasma is obtained by applying a voltage between the two electrodes.
FIG. 1 illustrates the distribution of the electric field E along a direct current glow discharge, the abscissa extending from the cathode (z =0) toward the anode (z=L, the length of the tube). In such a discharge, the most effective plasma production zone from the energy standpoint consists of the region R2, called the positive column, along which the axial electrical field adjusts itself so that the power given up by the electric field to the electrons e for maintaining the plasma allows exact compensation of radial plasma losses on the walls along the positive column, this so as to keep the discharge ignited. In the region of the cathode (called cathode region R1), on the other hand, there appears a very strong voltage drop (more than two or three hundred volts) which makes it possible to accelerate the ions i of the discharge toward the cathode, thus creating secondary electrons e2 which in their turn are injected into the gas with high energy, thus allowing ionization of the gas mixture.
A so-called negative glow region R3, where the electric field is practically null and which constitutes a diffusion space for the plasma and a drift space for secondary electrons not yet thermalized, is situated between the cathode region R1 and the positive column R2. Finally, region R4 located in proximity to the anode, where the electrons at the edge of the positive column R2 are accelerated toward the anode, is called the anode sheath. In the case of 50 or 60 Hz AC voltage, the electrodes are reversed at each alternation.
If a glow discharge is considered, the cathode region, where the electrode is polarized very negatively with respect to the anode, corresponds to a region where a great energy loss occurs, not usable for effective lighting. Indeed, in this region, positive ions are accelerated with an energy of several hundred electron volts (eV) onto the cathode, thus allowing emission of secondary electrons, accelerated in the opposite direction, which allow ignition and maintenance of the glow discharge and of its positive column. The consequence is that the difference in voltage between the anode and the cathode is found in large part in the region of the cathode (cathode drop).
In other words, though the cathode region allows ignition, then maintenance of a glow discharge, it constitutes a region of high energy loss dissipated in ion bombardment of the cathode. Besides this major shortcoming in terms of energy efficiency, glow discharge lamps (neon or compact fluorescent) have several shortcomings, among them unreliable ignition (especially at low temperature) of current lamps based on rare gas mixtures; the difficulty, even impossibility of igniting discharges containing plasma gases other than rare gas mixtures; deterioration of the electrodes due to their ion bombardment (cathode drop); reduced lifetime, particularly in the case of frequent, repeated extinguishing and lighting; the impossibility of controlling lighting using a dimmer; the presence of mercury in the gas mixture, which poses a toxicity and recycling problem.
One aim of the invention is to propose a glow discharge lamp making it possible to avoid the energy loss due to the intense bombardment of the cathode (or more generally of the electrodes in the case where a periodic voltage is applied). In fact, improving the efficiency of glow discharge lamps constitutes one of the major challenges to be met so as to significantly reduce worldwide consumption of electricity for lighting, which at present corresponds to 16% of electricity production. Another goal of the invention is to provide a glow discharge lamp which makes it possible to correct, to the extent possible, the other shortcomings and flaws mentioned above.